The present invention relates generally to fiber optic connectors and, more specifically, to the consolidation of fiber bundles utilized in such connectors.
The employment of fiber optic cables or light guides, also sometimes referred to as optical communications fibers, for the transmission of information bearing light signals is now an established art. Much development work has been devoted to the provision of practical low-loss glass materials and production techniques for producing glass fiber cables with protective outer claddings or jackets. The claddings make the cables resemble ordinary metallic-core electrical cables upon superficial external inspection. Obviously, if fiber optic cables are to be used in practical signal transmission and processing systems, practical connectors for the connection and disconnection of fiber optic cables must be provided.
Mateable connectors using either single or multiple fiber cables are subject to losses caused by traverse displacements, excessive axial gap distances, axial misalignment, broken fibers, reflective losses, chipped or scratched mating surfaces, and for multiple fiber bundles, low density fiber packing. Multiple fiber cable optical losses are minimized by consolidating the fiber optic bundle to produce a closed pack array, leaving only the spaces between the adjacent fibers and the fiber cladding as lost optical area. Traverse displacements caused by slop in the alignment socket in the connector, concentricity deviations of the contacts terminating the optical bundles, and individual fiber bundle diameter variations all result in light transmission losses. Coupled to concentricity deviations, rotational misalignments -- even when the contacts are keyed -- cause enough misalignment between fiber bundles to effectively randomize the mated fiber alignment and thus cause light transmission losses. The present invention is primarily concerned with the problem of consolidating the fibers in a fiber optic bundle to produce a closed pack array.
A common technique of obtaining a high density fiber bundle is the pushing of the bundle into a tight fitting contact or sleeve. Difficulties can be encountered in pushing the fibers into a restricting contact since some of the fibers will catch on the end of the contact and break. If the contact inside area is large enough so that the fibers in the bundle do not break, then the individual fibers remain somewhat loose so that high density packing is not achieved. Another technique for achieving fiber bundle consolidation is disclosed in the aforemention copending application Ser. No. 510,310. Such application describes a split tine contact which opens when the fiber bundle is inserted therein. Occasionally fibers in the bundle will protrude from the slits in the split tine thereby resulting in light transmission losses. Further, the use of a split tine contact creates an oval bundle configuration which must be keyed to obtain bundle-to-bundle rotational alignment. Keying of the contacts terminating aligned fiber bundles results in increased cost. Furthermore, as previously stated, even when contacts are keyed, rotational misalignments are sufficient to result in appreciable light transmission losses.
U.S. Pat. No. 3,846,010 to Love et al. discloses the use of an hexagonal ferrule for consolidating fibers in an optical fiber bundle. The patent states that the fibers in the bundle are retained in a closed pack array wherein the centers of the ends of any three contiguous fibers in the bundle are disposed at the vertices of an equilateral triangle. While theoretically maximum fiber consolidation can be achieved by this technique, in practice this is not the case. The reason is that natural tolerances such as fiber diameter variations, ferrule inside configuration variations, and ferrule profile eccentricity variations create closed pack imperfections which result in light transmission losses. Further, mating hexagonal ferrules require keying in order to minimize rotational misalignments. Even when the ferrules are keyed, losses can still result because of the problem of maintaining dimensional tolerances, particularly in the mass production of commercial connectors. Such problems with the hexagonal ferrule for terminating fiber optic bundles is not serious when the number of fibers is small and the fiber diameters are relatively large, on the order of 5 mils. However, in practice the hexagonal termination ferrule has not been entirely satisfactory for fiber optic bundles employing hundreds of fibers of diameters less than 2 mils. The minimum light transmission losses which have been reported as being achieved with the use of small fiber bundles has been on the order of 3 dB. Further, it will be appreciated that the cost of manufacturing the hexagonal ferrule would be significant and difficulties would be encountered in attempting to insert a fiber bundle into the ferrule without breaking peripheral fibers in the bundle.
Reference is also made to the aforementioned copending application Ser. No. 514,820 which discloses an hexagonal ferrule for terminating the end of a fiber optic bundle. The ferrule is formed of a heat-shrinkable metal which is shrunk down around the bundle end to consolidate the fibers. While greater consolidation may be obtained by this technique than that obtained by the technique disclosed in the Love et al patent, the construction of the heat-shrinkable metal ferrule still is not sufficient to compact the fibers in to the theoretical hexagonal pattern, nor enough to reduce optical transmission losses to levels required for some commercial applications.
We have attempted to obtain a high density packing of the fibers of a fiber optic bundle by using a plastic contact and permanently constricting the contact after the bundle has been inserted therein. A difficulty which is frequently encountered, however, during constriction of the contact is that the peripheral bundle fibers align in a compression hoop. The hoop is further supported by radially aligned fibers forming spokes. Since the fibers (normally quartz) are highly compression resistant, restriction beyond the fiber hoop is nearly impossible. This condition leaves internal fibers in the bundle in a low density, random pattern which will now allow optically efficient interconnections.
The purpose of the present invention is to overcome the problems and disadvantages discussed hereinabove with respect to the prior art termination techniques for fiber bundles, and to provide a method for fiber bundle consolidation which results in very high density packing of the fibers, even when very small diameter fibers are employed in the bundle, and is not significantly effected by rotational misalignment problems such as results from the use of split tine contact or hexagonal ferrule as described hereinabove.